Proposed protocols work clumsily in resolving the collisions occurred in dense RFID tag environment. QTDFS-ALOHA, a hybrid protocol which combines the query tree protocol and the dynamic frame slot ALOHA protocol, is proposed. In each frame of this protocol, only tags in the active set are allowed to answer in randomly selected slots. Afterwards, the population of tags in the active set is estimated. According to the identification accuracy required, the protocol may choose to continue the identification of the active set with delicately calculated frame length, or to divide the active set into multiple subsets using some binary prefix strings and identify each subset subsequently in the following frames. This process is performed recursively for all tag sets until the required accuracy is achieved. Proposed tag population estimation methods are summarized and evaluated. Numeric simulation verifies that this hybrid protocol outperforms other frame slot ALOHA based protocols.
Some fundamental limits on SAW RFID tag information capacity and collision resolution